[Redirecting to GHC users] Yes, GHC insists that you use 'case' when unpacking data constructor that bind existential type variables. http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions. html#existential-quantification (look at 7.4.1.4.3) It's not clear (to me) how to write the typing rule for pattern matching in lets when existentials are involved. Simon | -----Original Message----- | From: haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Dominic | Steinitz | Sent: 14 May 2006 09:03 | To: haskell@haskell.org | Subject: [Haskell] My brain just exploded | | What a great error message: | | qcgadt.hs:115:7: | My brain just exploded. | I can't handle pattern bindings for existentially-quantified constructors. | In the binding group | (Rep x) = uncompressRep s | In the definition of `uncompressRep': | uncompressRep (0 : 1 : 0 : s) = let (Rep x) = uncompressRep s in Rep | (RList x) | Failed, modules loaded: none. | | Here's the offending line: | | uncompressRep (0:1:1:s) = | let (Rep x) = uncompressRep s in | Rep (RList x) | | But this works: | | uncompressRep (0:1:0:s) = | let x = uncompressRep s in | case x of | (Rep y) -> Rep (RList y) | | Can someone explain this? | | Thanks, Dominic. | | | | _______________________________________________ | Haskell mailing list | Haskell@haskell.org | http://www.haskell.org/mailman/listinfo/haskell